1. Field of the Invention
The present invention relates to an electromagnetic flowmeter and, more particularly, to an electromagnetic flowmeter for relatively accurately measuring a flow speed or rate of a fluid to be measured regardless of a type of fluid and a diameter of a measuring pipe.
2. Description of the Related Art
An electromagnetic flowmeter generally measures a flow rate of a fluid by utilizing the Faraday's law.
In other words, when a magnetic field is applied to a conductive fluid flowing through a measurement tube, an electromotive force which is proportional to a flow rate of the fluid to be measured is generated.
By detecting the electromotive force, the flow speed and the flow rate of the fluid can be obtained.
In an industrial electromagnetic flowmeter, a fluid to be measured is usually a fluid containing an electrolyte. When a fluid contains an electrolyte, a kind of a battery is formed by electrochemical phenomenon on electrodes. An output voltage of the battery is larger than a voltage of the signal proportional to the flow rate. Therefore, an accurate electromotive force cannot be measured. For this reason, the electromagnetic flowmeter using the DC magnetic field cannot use for measuring a flow rate of a fluid containing an electrolyte.
In order to measure the flow rate of a fluid containing an electrolyte, an electromagnetic flowmeter for exciting a magnetic field generation coil by using a commercial AC power source is developed. In this electromagnetic flowmeter, however, variations in output signal levels caused by variations in zero point of the output signal may occur due to an AC phenomenon.
In order to eliminate the drawbacks of these conventional electromagnetic flowmeters, a flowmeter for exciting a magnetic field generation coil by using a square (rectangular) wave signal was developed. In this electromagnetic flowmeter, an electromotive force obtained in a stable area of a magnetic flux density B of a square wave magnetic flux is measured to obtain a flow rate of a fluid to be measured. In this electromagnetic flowmeter, an excitation frequency is low. Therefore, this electromagnetic flowmeter, however, is susceptible to noise having frequencies close to that of the magnetic field, e.g., noise called (1/f) noise and aliasing noise. It is possible to prevent noise by increasing an excitation frequency. When the frequency is increased, a rise time of a magnetic flux is undesirably prolonged due to an iron loss and the like. For this reason, an area where the magnetic field is kept stable is shortened. Therefore, the square wave magnetic flux behaves like an AC magnetic flux. The AC phenomenons which affect measurement precision are impaired. This drawback becomes more conspicuous in accordance with the increase of the diameter of the measurement pipe.
In order to eliminate the above drawback, a dual frequency excitation type electromagnetic flowmeter is proposed in "NEW INTELLIGENT MAGNETIC FLOWMETER WITH DUAL FREQUENCY EXCITATION", ISA, 1988-Paper #88-1566. In this electromagnetic flowmeter, a magnetic field excitation coil is driven by a signal obtained by superposing square wave excitation signals of a low frequency (about 6 Hz) and a high frequency (about 100 Hz) on each other. The electromotive force signal detected by the electrodes is separated into a signal induced by low-frequency excitation using a filter and a signal induced by high-frequency excitation. The separated signals are processed to obtain a flow rate.
In the magnetic flowmeter with dual frequency excitation in the above literature, a rise time of a magnetic flux of the high-frequency excitation signal is prolonged due to an iron loss. For this reason, an area where the magnetic flux is kept stable is shortened, the magnetic flux obtains AC characteristics and loses square wave characteristics. Therefore, an AC phenomenon for varying the zero point of the output signal also occurs. The drawback of the high-frequency excitation signal becomes more conspicuous in accordance with the increase of the diameter of the measuring pipe. In addition, unless a difference between the frequencies of the two excitation signals is large, the resultant flow rate signal cannot be accurately separated into two signals by using a filter.
It is, therefore, difficult for these conventional electromagnetic flowmeters to accurately measure a flow rate of a fluid to be measured.